1. Field of the Invention
The present invention relates to a structure obtained by mounting various electronic components on a printed circuit board (it will be usually referred to as a PCB) by soldering and a method for manufacturing the same. More particularly, it relates to a structure in which the electronic components are mounted on the PCB by reflowing, and a method for manufacturing the same.
2. Description of the Related Art
Hitherto, soft soldering is employed for fixedly mounting electronic components on a PCB. A description of an example of the method of mounting electronic components by the use of a soft solder is provided hereinbelow with reference to FIG. 1. Here, the description of a case where soldering is applied by the known reflowing technique to both surfaces of the PCB, respectively, will be provided.
First, a metal mask provided with apertures at only the corresponding positions to land portions of the PCB is used for printing solder paste on the land portions (Step 101). Subsequently, electronic components such as chips, QFP (Quad Flat Package), SOP (Small Outline Package) and so on are mounted on the PCB, so that the electric terminals and leads of these electronic components are mounted onto the printed solder paste (Step 102). Thereafter, the PCB mounting thereon the electronic components is urged to pass through a high-temperature reflowing furnace so as to fuse the solder paste thereby soldering the electrodes of the electronic components to the land portions of the PCB (Step 103).
The described process permits completion of the mounting of the electronic components onto one of both surfaces of the PCB. Therefore, the PCB is subsequently reversed so that the other surface mounting thereon no electronic components is held upward (Step 104).
Subsequently, similarly to the described Steps 101 and 102, the printing of the solder paste (Step 105) and mounting of the electronic components (Step 106) are implemented. Thereafter, the components having their electric leads are inserted in the through-holes (Step 107). Then, similarly to the Step 103, the PCB is urged to pass through the furnace so as to complete the soldering of the components (Step 108).
Finally, some electronic components that are not able to withstand a high temperature in the reflowing furnace are subjected to the process of manually soldering these components to thereby complete mounting of the electronic components onto the PCB (Step 109).
In the above-described mounting method of the electronic components according to the known technology, solder paste containing therein solder of tin and lead (Sn—Pb) is generally used. However, since the Sn—Pb solder contains therein lead (Pb) that is a toxic heavy metal, unless electronic appliances after usage are adequately put on the discard, there has occurred such a problem that an adverse affect is provided on the global atmosphere. Taking this into consideration, in recent years, in order to solve the described problem to thereby prevent any environmental pollution from occurring beforehand, employment of a Pb-free solder containing therein no lead component is eagerly desired.
In this respect, a tin and silver (Sn—Ag) type solder is typically known as the Pb-free solder. Since the property of the silver (Ag) is stable, when the Sn—Ag type solder is used for mounting the electronic components in lieu of the Sn—Pb type solder, it can ensure identical degree of reliability with the conventional mounting method. Nevertheless, compared with the melting point of the Sn—Pb type solder being about 183° C., the melting point of the Sn—Ag type solder higher being about 220° C. Therefore, the conventional mounting apparatus and method having employed the Sn—Pb type solder cannot be directly applied when the Sn—Ag type solder is employed.
If the Sn—Ag type solder having the melting point of as high as 220° C. is fused in the reflowing furnace to implement the soldering of the electronic components, the temperature of the components could occasionally be more than 240° C. Since the heatproof temperature of the electronic components is usually approximately 230° C., when the Sn—Ag type solder is employed for mounting the electronic components, such a problem will be encountered that the heatproof temperature of various sorts of electronic components should be raised.
There is another Pb-free solder different from the Sn—Ag type solder having the above-mentioned high melting point, i.e., a tin-Zinc (Sn—Zn) type solder. Since the melting point of the Sn—Zn type solder is approximately 197° C., when the Sn—Zn type solder is employed for the mounting of electronic components, the conventional equipment and electronic components can be directly employed without any changes to them.
Nevertheless, when the Sn—Zn type solder is compared with the conventionally employed Sn—Pb type solder, there are problems such that the Zinc (Zn) is apt to be oxidized, and the wettability of the Sn—Zn type solder is rather poor. Accordingly, when the mounting of the electronic components is conducted by the direct use of the conventional equipment and the conventional mounting method, it cannot be ensured that the mounting reliability is equivalent to the conventional one.
At this stage, a description of comparison between the mounting of electronic components implemented by the use of the Sn—Pb type solder and that implemented by the use of the Sn—Zn type solder will be provided below.
FIGS. 2A through 2C are schematic cross-sectional views illustrating a manner in which the lead of a QFP that is an electronic component is electrically connected to the land of a board by the employment of the Sn—Pb type solder. FIG. 2A illustrates the lead of the QFP before it is connected to the land of the board, FIG. 2B illustrates the lead of the QFP after it is connected to the land of the board, and FIG. 2C illustrates, in an enlarged scale, the fillet of the Sn—Pb type solder provided at the extreme end of the lead illustrated in FIG. 2B.
As illustrated in FIGS. 2A through 2C, a wiring pattern is produced on board 104 by a copper foil or the like, and a part of the produced pattern is formed in land 103 to which the electric lead terminals of various sorts of electronic components are electrically connected. Onto land 103, a solder paste containing therein the Sn—Pb type solder for electrically jointing and connecting the terminals of the electronic component is printed. At this stage, the wiring pattern on board 104 is covered by a layer of an insulative resist, and land 103 is formed by removing the resist layer at a portion of the wiring pattern. However, it is noted that the instant figures and the other drawings are schematic views for omitting the resist layer.
Here, the above-mentioned printing process of the solder paste will be described with reference to FIGS. 3A through 3C.
First, as illustrated in FIG. 3A, printing mask 150 is mounted and positioned onto board 104, so that respective apertures 150a of printing mask 150 are corresponding to respective lands 103. Subsequently, a predetermined amount of solder paste 151 is placed on printing mask 150 mounted on board 104, and as illustrated in FIG. 3B, squeegee 152 is employed for urging solder paste 151 to perform rolling and moving over the surface of printing mask 150 from an end thereof to the opposite end.
In the course of rolling and moving of solder paste 151 over the surface of printing mask 150, paste 151 is impressed into respective apertures 150a by means of squeegee 152, so as to fill up apertures 150a. Then, as illustrated in FIG. 3C, when printing mask 150 is separated away from board 104, a predetermined amount of solder paste 151 is printed on each of lands 103 of board 104, and thus the printing process of the solder paste is terminated.
Thereafter, by way of the above-mentioned component mounting process and reflowing process, when lead 101a of QFP 101 is soldered to land 103 (refer to FIG. 2B), fillet is formed at front and rear ends of lead 101a laid on land 103 by the action of the surface tension of Sn—Pb type solder 102a per se (refer to FIG. 2C). At this time, lead 101a is covered by a sufficient amount of the solder fillet, i.e., generally, one third or more in thickness of the lead is covered. Thus, the strength of joint of lead 101a to land 103 can be acquired.
FIGS. 4A through 4C are schematic cross-sectional views illustrating a manner in which the lead of a QFP that is an electronic component is electrically connected to the land on the board by the use of a Sn—Zn type solder which is different from the example of FIGS. 2A through 2C.
FIG. 4A illustrates the lead of the QFP in a state before it is connected to the land of a board, FIG. 4B illustrates the lead of the QFP in a state after it is connected to the land of a board, and FIG. 4C illustrates, in an enlarged scale, the fillet of the Sn—Zn type solder provided at the extreme end of the lead shown in FIG. 4B.
In the present example, when lead 101a of QFP 101 is soldered to land 103 (refer to FIG. 4B) by way of the afore-mentioned printing process, component mounting process, and reflowing process, the fillet is formed, by the surface tension of Sn—Zn type solder 102b per se, at the front and rear ends of lead 101a laid on land 103 (refer to FIG. 4C). Nevertheless, since Sn—Zn type solder 102b is poor in its wettability as described before, lead 101a cannot be covered by a sufficient amount of solder fillet (generally, equal to or more than one third of the thickness of the lead), and therefore sufficient strength of joint of lead 101a to land 103 cannot be acquired. As a result, either a defective joint or a breakage of the joint could occur between lead 101a and land 103.
Further, FIGS. 5A and 5B illustrate a manner in which an electric lead of a connector component that is an electronic component is connected to the land of a board according to the prior art by the employment of the Sn—Pb type solder. FIG. 5A illustrates the lead of the connector component in a state before it is connected to the land on the board, and FIG. 5B illustrates the lead of the connector component in a state after it is connected to the land of the board.
In the present example, similar to the example shown in FIGS. 2A through 2C, a wiring pattern is produced on board 204 by the use of a copper foil or the like, and a portion of the produced pattern is formed as a land 203 to which the lead terminals of various electronic components are connected to form electric joints. On such land 203, solder paste containing therein Sn—Pb type solder 202a for connecting the terminals of the electronic components is printed.
By way of the afore-described printing process, component mounting process, and reflowing process, when lead 201a of connector component 201 is soldered to land 203 (refer to FIG. 5B), fillets are formed, by the surface tension of Sn—Pb solder per se, at the front and rear ends of lead 201a laid on land 203. Thus, similar to the example of FIGS. 2A through 2C, the joint strength of lead 201a to land 203 can be acquired.
FIGS. 6A and 6B, differing from FIGS. 5A and 5B, are schematic cross-sectional views illustrating a manner in which the lead of a connector component that is an electronic component is connected to the land of a board by the employment of the Sn—Zn type solder. FIG. 6A illustrates the lead of the connector component in a state before it is connected to the land of the board, and FIG. 6B illustrates the lead of the connector component in a state after it is connected to the land of the board.
In the present example, when lead 201a of connector component 201 is soldered to land 203 (refer to FIG. 6B) by way of the afore-described printing process, component mounting process and reflowing process, fillet is formed by the surface tension of Sn—Zn type solder 202b per se at the front and rear ends of lead 201a laid on land 203. Nevertheless, since Sn—Zn type solder 202b is poor in its wettability as described before, lead 201a cannot be covered by a sufficient amount of solder fillet (generally, equal to or more than one third of the thickness of the lead), and accordingly the strength of joint of lead 201a to land 203 cannot sufficiently be acquired.
Also, in the case of connector component 201, since a difference in the height between the lower face of the body of connector component 201 and the lower face of lead 201a is generally small, and since a portion of that body per se lies above and covers a portion of land 203, when connector component 201 is mounted on board 204, as shown in FIG. 6B, a part of Sn—Zn type solder 202b might unfavorably attach to the lower face of the body of connector component 201.
In the case of the Sn—Pb type solder (refer to FIGS. 5A and 5B), as the wettability thereof is rather high, the solder existing below the body of the connector component would move to a position adjacent to the rear end of the lead for forming the solder fillet. However, in the case of the Sn—Zn type solder, the wettability thereof is low or poor, and accordingly the solder existing below the body of the connector component does not move too much and stays there resulting in that it is sandwiched between the body of connector component 201 and land 203.
FIG. 7 is a perspective plan view of the constitution as shown in FIG. 6B.
As described above, due to the low wettability of the solder, Sn—Zn type solder 202b sandwiched between the body of connector component 201 and land 203 would overflow into a space between the neighboring lands 203, and occasionally a bridge 205 of the solder might be formed so as to short-circuit between the neighboring lands 203. To the contrary, in the case of the Sn—Pb type solder, the wettability thereof is better, and accordingly the solder bridge is not formed at all.
Thus, if the solder joint of the electronic component by the employment of the Sn—Zn type solder is applied in a manner similar to that of application of the solder joint by the employment of the conventional Sn—Pb type solder, it has been clarified that some defects as described above must be met by the cause of the low or poor wettability of the Sn—Zn type solder.